This invention relates to a power source supplying circuit and a power source supplying method and, more particularly, to a power source supplying circuit and a power source supplying method which are capable of supplying a constant voltage to a load although variation of a load current is large.
As is well known in the art, power source supplying circuits of the type described are classified into two types, namely, first and second types in the manner which will presently be described. Specifically, the first type of the power source supplying circuits are a type where there is no charging circuit while the second type of the power source supplying circuits are a type comprising a charging control circuit. At first, the first type will be described. The second type will be later in the following.
In the manner which will later be described in conjunction with FIG. 1, a conventional power source supplying circuit of the first type has an external input/output connector power supply terminal, an external input/output connector ground terminal, a battery connection power source terminal, and a battery connection ground terminal. The battery connection power source terminal is connected to a portable radio apparatus serving as a load. A backflow check diode is disposed between the external input/output connector power source terminal and the battery connection power source terminal. Specifically, the backflow check diode has an anode electrode connected to the external input/output connector power source terminal and a cathode electrode connected to the battery connection power source terminal. As described above, the first type of the conventional power source supplying circuit does not carry out a power source control.
In the manner which will later be described in conjunction with FIG. 2, it is impossible for the conventional power source supplying circuit of the first type to supply the portable radio apparatus with a stable voltage. This is because a supply voltage drops by one corresponding to a voltage drop caused by a load current.
In the manner which will later be described in conjunction with FIG. 3, a conventional power source supplying circuit of the second type has not only the external input/output connector power supply terminal, the external input/output connector ground terminal, the battery connection power source terminal, and the battery connection ground terminal but also a battery connection battery presence or absence detection terminal. A charging control transistor is disposed between the external input/output connector power source terminal and the backflow check diode. A current limiting resistor is disposed between the backflow check diode and the battery connection power source terminal. Specifically, the charging control transistor consists of a P-channel metal oxide semiconductor field effect transistor (MOSFET). The P-channel MOSFET has a source electrode connected to the external input/output connector power source terminal and a drain electrode connected to the anode electrode of the backflow check diode. The cathode electrode of the backflow check diode is connected to an end of the current limiting resistor which has another end connected to the battery connection power source terminal.
The conventional power source supply circuit of the second type further comprises a current detection circuit for detecting a current flowing in the current limiting resistor and a charging control circuit. The charging control circuit is supplied with a current detected signal and a battery presence and absence detected signal from the current detection circuit and the battery connection batter presence and absence detection terminal, respectively. Responsive to the current detected signal and the battery presence and absence detected signal, the charging control circuit controls the charging control transistor. The charging control circuit supplies a control signal to a control terminal (gate electrode) of the charging control transistor.
In the manner which will later be described in conjunction with FIG. 4, inasmuch as the backflow check diode and the current limiting resistor serves as a voltage drop arrangement. As a result, it is difficult for the conventional power source supplying circuit of the second type to supply the portable radio apparatus (the load) with a stable voltage.
In addition, various prior arts related to this invention are already known. By way of example, Japanese Utility Model Registration No. 2,571,356 or JP-Y2571356 discloses an absolute displacement measurement equipment comprising a power source voltage control circuit. In the power source voltage control circuit described in JP-Y 2571356, a voltage supplied from a solar cell is supplied to an EEPROM acting as a load via a first backflow check diode and two paths. A first path supplies the load with the voltage through a small-capacity capacitor for stabilizing a power source voltage. A second path supplies the load with the voltage through a constant current circuit, a charging circuit, a large-capacity capacitor, and a second backflow check diode. In addition, the power source voltage control circuit is provided with a voltage detection circuit for detecting a voltage between both ends of the large-capacity capacitor to produce a voltage detected signal which is supplied to the load and the charging circuit.
However, in the manner as the conventional power source supplying circuit of the first type, it is impossible for JP-Y2571356 to supply the load with a stable voltage caused by a voltage drop in the backflow check diode.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Show 64-30, 430 or JP-A 1-030430 discloses a power source circuit which is capable of preventing overcurrent even when a capacitor with large capacitance is employed, by providing a resistor for limiting charging current of capacitor, a bypass diode and a diode for preventing reverse charging of capacitor. The power source circuit disclosed in JP-A 1-030430 comprises a series circuit, a capacitor, and a bypass diode. The series circuit is connected between a DC power source and a load. The series circuit consists of a backflow check diode, a current limiting resistor, and a back-charging check diode. The capacitor is connected between a ground and a connection point of the current limiting resistor and the back-charging check diode. The capacitor supplies the load with an electric power on instantaneously breaking of the DC power source and on decreasing of a voltage. The bypass diode is in parallel connected to the series circuit. The bypass diode supplies the load with the electric power when the DC power source is normal or when the capacitor is initially charged.
With this structure, even when the capacity of the capacitor is increased, initial charging current of the capacitor is limited by the current limiting resistor. Consequently, overcurrent does not flow into the backflow check diode and voltage drop of the DC power source due to overload causes no bad influence onto the load. Inasmuch as the current limiting resistor is inserted, long time is required until charging voltage of the capacitor increases to a level for enabling operation of the load during initial charging stage. Consequently, the bypass diode is connected to supply power normally from the DC power source through the bypass diode to the load.
However, it is difficult for JP-A 1-030430 to supply the load with a stable voltage because of voltage drop in the backflow check diode, the current limiting resistor, the back-charging check diode.
Furthermore, Japanese Unexamined Patent Publication of Tokkai No. Hei 8-317,571 or JP-A 8-317571 discloses a charge circuit for a secondary battery, which prevents the reverse flow of a current from a battery to the side of a charge circuit, and also shortens the charge time by enabling the accurate measurement of battery voltage, and further does not deteriorate the battery due to overcharge. In the charge circuit for the secondary battery disclosed in JP-A 8-317571, a reverse flow preventive (backflow check) diode is inserted between a secondary battery and a charge current control circuit which controls the charge current to be supplied to the secondary battery, on the basis of the output of a voltage detecting circuit for detecting the terminal voltage of the secondary battery. An FET is inserted between the secondary battery and the voltage detection circuit. A switch control circuit performs the control to put the FET in current-conduction state when the power source for charge is turned on and to put it in no current-conduction state when the power source for charge is turned off.
The above-mentioned JP-A 8-317571 may disclose the invention related to the charge circuit for the secondary battery. If the battery regards as a load in JP-A8-317571, it is difficult to supply the load (the battery) with a stable voltage because of voltage drop in the reverse flow preventive (backflow check) diode.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Hei 10-243,553 or JP-A 10-243553 discloses a battery-driven electronic equipment and a control method therefor, which are capable of miniaturizing an external power supply with a function of charging an internal battery maintained, by controlling a charging circuit according to the presence or absence of connection with the external power supply and the state of the operation of electronic equipment. In the battery-driven electronic equipment disclosed in JP-A 10-243553, a system-off signal generated by a power switch is supplied to a power supply control circuit. The power supply control circuit sends a control signal to a constant voltage control circuit. The constant voltage control circuit turns off a constant voltage circuit (charging circuit), and a system is thereby turned off. Thereafter, the power supply control circuit sends a control signal to a charge control circuit, and the charge control circuit turns off the charging circuit to charge a battery. Thereby, an external power supply of small capacity can be used with a charging function maintained, and thus the external power supply can be miniaturized in electronic equipment.
Inasmuch as a voltage is supplied to the system from the external power supply through the constant voltage circuit in JP-A 10-243553, a predetermined voltage may be supplied to the system. However, in a case where there is a voltage drop arrangement such as a backflow check diode, a current limiting resistor, or the like, JP-A 10-243553 never discloses a concrete embodiment regarding how to supply a constant voltage.
Japanese Unexamined Patent Publication of Tokkai No. Hei 9-130,981 or JP-A 9-130981 discloses a charge control device which is capable of always holding charged voltage of a battery at a charge stop voltage. In the charge control device disclosed in JP-A 9-130981, when voltage of a battery decreases to set a mobile terminal such as a portable telephone set, a portable radio terminal, or the like to a charger for charging, a microcomputer turns on first and second changeover switches and turns off a third changeover switch, so that a charge current is supplied to the battery and the mobile terminal, that is, a portable device control circuit through a protecting diode by an external control circuit. When completion of charging is detected by the microcomputer, it turns off the first and the second changeover switches also turns on the third changeover switch, the current by the external input voltage is supplied only to the mobile terminal, that is, the portable device control circuit, so as to suppress consumption of the battery.
In the above-mentioned JP-A 9-130981, the microcomputer merely carries out a constant voltage charging control operation to the battery by controlling turning on and off of the first through the third changeover switches. In other words, JP-A 9-130981 never describes regarding how to hold a voltage constant in a case where a load current of a load varies.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Hei 9-190,231 or JP-A 9-190231 discloses a power source device which is capable of outputting highly precise voltage. According to JP-A 9-190-231, the power source device comprises a voltage control circuit of a shunt regulator system that is connected to a power supply circuit through a resistor. The voltage control circuit is connected in parallel with a load.
However, a load variation is not taken into account at all in the above-mentioned JP-A 9-190231 in the similar manner as JP-A9-130981. That is, JP-A 9-190231 never discloses nor teaches regarding how to hold a voltage constant in a case where a load current of the load varies.